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ARROW-5195: [C++] Detect null strings in CSV string columns

By default, when converting CSV to a string column, all CSV values are considered valid.

Add an option so that strings such as "N/A" etc. are considered null.

Author: Antoine Pitrou <antoine@python.org>

Closes #4188 from pitrou/ARROW-5195-csv-allow-null-strings and squashes the following commits:

9dfff4de4 <Antoine Pitrou> ARROW-5195:  Detect null strings in CSV string columns